Paper comprising polybenzazole or precursor thereof

ABSTRACT

The invention relates to a paper comprising at least one of a fiber, pulp, fibril, floc, and fibrid having a polybenzazole structure with a repeating unit of formula (I) and/or (II) 
     
       
         
         
             
             
         
       
         
         
           
             or its precursor structure with a repeating unit of formula (III): 
           
         
       
    
     
       
         
         
             
             
         
       
         
         
           
             wherein Ar 1  and Ar 2  are independently a para or meta aromatic group having 4 to 12 carbon atoms, X and Y are the same or different and selected from O, S, and NH; and n is 0 or 1, and wherein the paper is free or essentially free of non-extractable phosphorus compound. The paper is particularly suitable for making an electrical insulation material, a honeycomb structure, or a constructive material.

The invention relates to a paper comprising at least one of a fiber,pulp, fibril, floc, and fibrid containing a polybenzazole structure or apolybenzazole precursor structure. The invention further pertains to amethod for making such papers and to the use thereof.

It has described in EP 07008742 that fiber, pulp, fibril, or fibridhaving superior properties, including mechanical properties, can beobtained by a process in which an optical anisotropic dope, containing ahigh concentration of a high molecular weight aromatic polyamide havinga substituent such as a hydroxy, thiohydroxy, or amine group in anacidic solvent, is applied using a wet air gap spinning process, a jetspinning process, or any other conventional method to obtain a fiber,pulp, fibril, or fibrid, which are then heat treated.

The present invention relates to paper comprising at least one of afiber, pulp, fibril, floc, and fibrid having a polybenzazole structurewith a repeating unit of formula (I) and/or (II)

or its precursor structure with a repeating unit of formula (III):

wherein Ar¹ and Ar² are independently a para or meta aromatic grouphaving 4 to 12 carbon atoms, X and Y are the same or different andselected from O, S, and NH; and n is 0 or 1, and wherein the paper isfree or essentially free of non-extractable phosphorus compound,wherein the paper contains less than 0.15 wt % of non-extractablephosphorus compound.

Papers made of fibers having a polybenzazole structure are known in theart, for instance from JP 10 096175, JP 2001 248091, and WO 2007/076332.JP 10 096175 relates to non-woven sheets rather than to paper.Furthermore, these sheets and papers have been made from fibers that arespun from polyphosphorus spinning dopes. Therefore, these papers containa considerable amount of non-extractable phosphorus compound, since eventhe most sophisticated methods for removing polyphosphorus acid leavesat least 0.25 wt %. Normal commercial procedures leave about 0.4 wt % ofpolyphosphorus acid in the fiber (see for instance, Hu X. B. and LesserA. J.; Abstracts of Papers of the American Chemical Society 2004,227:U562-U562).

The terms “para” and “meta” relate to the positions of the two aminogroups or the two carbonyl groups at the aromatic ring. If Ar¹ and/orAr² contain annelated aromatic rings there are formally no para and metapositions, but the corresponding positions are called pseudo-para andpseudo-meta positions, which are included in the definition of “para”and “meta”.

The paper is free or essentially free of non-extractable phosphoruscompound, which means that the paper contains less than 0.15 wt % ofnon-extractable phosphorus compound and preferably no non-extractablephosphorus compound at all.

The present fibers, pulp, fibrils, floc, or fibrils are manufactured bya method comprising the steps of spinning or extruding a dope andsolidifying it to a coagulation liquid, and then subjecting the obtainedfiber as was described in EP 07008742.

The invention also relates to a precursor paper, which as such hasexcellent properties and therefore can be used as such. This precursorpaper contains a polybenzazole precursor having the repeating unitexpressed by formula (III):

wherein Ar¹ and Ar² are independently an aromatic group having 4 to 12carbon atoms, Ar¹ and Ar² have the para or meta configuration, X and Yare the same or different and selected from O, S, and NH, and n is 0 or1.

Examples of Ar¹ are phenylene, naphthalenediyl, and bivalentheteroaromatic groups. Ar¹ may be substituted with hydroxy and/orhalogen groups.

Ar¹ is preferably selected from

Ar² is a tri- or quadrivalent aromatic group with 4-12 carbon atoms.Examples of Ar² are benzenetri- or tetrayl, naphthalenetri- or tetrayl,diphenyltri- or tetrayl, and tri- or quadrivalent heterocyclic group canbe listed as Ar², These Ar² moieties may be substituted with a hydroxyand/or halogen group.

Ar² is preferably selected from:

The benzene group is the most preferred Ar² group.

In a preferred embodiment Ar¹ is para- or meta-phenylene:

and Ar² is

wherein X and Y are O, and the straight lines represent a bond.

In addition to the above polybenzazole the fiber may also be a copolymercontaining repeating units expressed by formula (IV)

In formula (III), the Ar¹ groups have independently the previously givenmeanings. The preferred Ar¹ group is para- or meta-phenylene.

The polybenzazole preferably comprises 40 to 100 mole % of the repeatingunit expressed by formula (I) and/or (II) with 60 to 0 mole % of therepeating unit expressed by formula (IV), to a total of 100 mole %.

The polybenzazole more preferably comprises 60 to 100 mole % of therepeating unit expressed by formula (I) and/or (II) with 40 to 0 mole %of the repeating unit expressed by formula (IV), to a total of 100 mole%.

Since X is an oxygen atom (—O—), sulfur atom (—S—), or imino group(—NH—), the polybenzazole which can be obtained form the polymerprecursors contains imidazole, thiazole, and/or oxazole rings.

The polybenzazole precursor containing one or more of the followingrepeating units is especially preferred.

Methods for making these polymers, and for making fiber, pulp, fibril,floc or fibrid thereof are disclosed in European patent application no.EP 07008742, which is incorporated by reference.

Although PBO paper is known in the art, i.e. as mentioned in U.S. Pat.No. 6,890,636, such paper inherently contains substantial amounts ofphosphoric acid which was used as spin dope for making fiber, and whichcannot completely be removed. The PBO paper of this invention containsless than 0.15 wt % of non-extractable phosphorus compound (i.e. mainlyphosphoric acid), preferably much less such as less than 30 ppm, andmost preferably none or virtually none of phosphorus compound (when thespin dope does not contain any phosphoric acid). Because it is knownthat traces of phosphoric acid may decompose PBO fibrous materials,leading to substantial loss of paper strength, it may be of utmostimportance to make PBO paper that is free or at least substantially freeof phosphoric acid, if such paper should maintain its strengths for longperiods. The unique method for making the PBO paper of this inventionresides in a method wherein the ring-closed PBO structure is obtainedfrom an open precursor structure still having OH, SH, or NH₂ groups.These hydrophilic groups allow the precursor to dissolve in hydrophilicsolvents such as water, alcohol, water-alcohol mixtures, and the like.Whereas PBO can practically only be dissolved in phosphoricacid-containing spin dopes, the present precursors can form spin dopesin said hydrophilic solvents, without using any phosphoric acid. Suchspin dopes will lead to fiber, pulp, fibril, floc or fibrid that iscompletely or virtually completely free from phosphorus compound. PBOpaper having less than 0.15 wt % phosphorus compound is unknown. Theknown PBO papers have been made from PBO-polyphosphorus acid-containingspin dopes, leading to paper having (much) more than 0.15 wt %non-extractable phosphorus. Although it is usually not preferred, smallamounts of phosphorus acid or other phosphorus compounds can be added tothe spin dope, leading to papers having minor amounts (i.e. less than0.15 wt %) of phosphorus. The amount of phosphorus present in the papercan easily be measured by using standard methods such as by spectroscopyor titration.

The papers of this invention may include combinations of fiber, pulp,fibril, floc or fibril, such as fibrids and floc. The papers of theinvention can be made by conventional papermaking processes, whichprocesses allow adding common additives and auxiliary materials to thematerial for making paper, such as pigments, binders, silicates,fillers, and other additives. The paper such obtained may be processedfurther such as by applying known calendaring methods to further enhancethe density of the paper.

The terms “fibers, pulp, fibrils, floc, and fibrids” are well known inthe field, and for instance can be found in Textile Terms andDefinitions, 2^(nd) Ed, 1955. The term “fibrids” refers to non-granularfilm-like particles. The fibrids have an average length of 0.2 to 1 mmwith a length-to-width aspect ratio of 5:1 to 10:1. The thicknessdimension is on the order of a fraction of a micron. Such fibrids, whenfresh, are used wet and are deposited as a binder physically entwinedabout the floc component of the paper. Fresh fibrids andpreviously-dried fibrids can be used in paper of this invention.

The term “floc” refers to short fibers, typically having a length of 2to 12 mm and a linear density of 1-10 decitex. The floc can be fresh orit can be previously-dried. If fresh, it has not before been used in anyproduct.

Paper pulp may comprise floc and fibrids, generally, in amounts of about50-60%, by weight, fibrids and 40-50%, by weight, floc. Even aftercomminuting and milling, the floc in aramid paper pulp is bound, to someextent, by the fibrids. The fibrids, being in a dried state, are boundtogether or collapsed and less useful as binder material than the fresh,never-dried, fibrids; but, due to their random, rigid, irregular, shape,contribute an increased porosity to the final paper structure. Forpurposes of this invention, those fibrid and floc components taken fromdried papers may be called previously-dried fibrids and previously-driedfloc.

Dried paper sheets containing polybenzazole precursor can also beprocessed through a high speed milling machine, such as a turbulent airgrinding mill known as a Turbomill or an Ultra-Rotor, and then wetrefined. Turbulent air grinding mills are preferred for comminutingpapers which have been calendered; but the grinding mills result inslightly shortened fiber lengths. Paper of this invention using paperpulp with shortened fiber lengths exhibits slightly reduced wet strengthand a tendency to worsen paper machine continuity.

The paper made from the polybenzazole precursor material can be used assuch. It has excellent properties as will further be demonstrated in theexperimental part. However, the properties of this paper can easily bechanged or improved by functionalizing at least part of the free XH andYH groups, such as OH groups. These free groups are able to react withmonomers and polymers having reactive groups, such as esters,isocyanates, epoxides, and other functionalizing agents to give acovalent bond between X and/or Y and the functionalizing agent. If partof the free XH and YH groups is functionalized these papers can also beheat treated to convert the polymer precursor by a cyclizing process toring-closed PBO polymers, thereby obtaining functionalized PBO paper.

Functionalizing of all or part of the XH and YH groups can be done invarious phases of the papermaking process. Thus it is possible tofunctionalize (part of) the XH en YH groups in the monomer

followed by polymerization with the monomer ClOOC—Ar¹—COOCl. Thefunctionalized polymer can then be treated in any of the above describedmanners to obtain the paper of the invention.

Functionalizing can also be performed on the precursor polymer or thepolybenzazole, as obtained by polymerization of the monomers. Thesepolymers may contain XH and/or YH groups which can be functionalize byreaction with a functionalizing agent. The polymer can be functionalizedin any of the stages during the process of making paper. Thus thepolymer can be functionalized just after polymerization of the monomers,but it can also be functionalized in the form of a fiber, pulp, fibril,floc, or fibrid, or after the paper has been made. In the latter methodsin most cases only the outer surface of the fiber, pulp, fibril, floc,or fibrid can easily be functionalized, which can be an advantage ifonly partial functionalization is desired.

In this manner papers can be made of which the properties have beenchanged by functionalization, such as coloring, smoothening, makingwater repellant, increasing or decreasing the conductivity, and makingfire resistant paper.

Because the polymer precursor has been synthesized and spun fromsolutions that may be free from phosphorus compounds, the PBO obtainedcan also be free of phosphorus compounds. It is a further advantage thatit is no longer required to make the paper from almost insoluble PBOpolymers, but the papermaking process can be performed with readilysoluble polymer precursors, and conversion to PBO takes place afterformation of the paper.

In general the papers from this invention exhibit lower porosity thanPPTA papers making them very suitable for electrical applications suchas in electrical insulation material. The papers are further suitablefor application in honeycomb structures and in constructive materials.

The papers of the present invention, both for PBO precursor-containingpapers and PBO papers, have a much higher strength than known papers, asshown by EAB (elongation at break) and TI (tenacity index) data. Forinstance, the present papers are superior to PPTA paper and even toNomex®, which is considered the strongest paper known until now.

The extreme strength of the present papers makes it possible to produceextreme thin papers. The papers of this invention also have superiorheat stability compared to PPTA paper and Nomex®.

Because of the unusual strength of the present papers, papers having agrammage between 1 and 16 g/m² can be made. The term “grammage” is ametric measure of paper weight based on the same square meter sheet ofpaper, regardless of paper grade.

The present invention will be explained more specifically by thefollowing embodiments. However, the present invention is not limited tothese embodiments.

General:

These results were obtained with the polymer precursor having thefollowing repeating unit:

and with the corresponding ring closed polymer having the repeatingunit:

wherein Ar¹=para-phenylene and Ar²=diphenylene

Abbreviations:

-   NMP=N-methylpyrrolidone-   DHB=dihydroxybenzidine (4,4′-diamino-3,3′-dihydroxydiphenyl)-   TDC=terephthaloyl dichloride-   PPD=para-phenylenediamine-   PPTA=para-phenyleneterephthalamide

EXAMPLE 1 Polymerization to Polybenzoxazole Precursor

2.25 L of NMP/CaCl₂ and 1.75 L of NMP together with pre-dried DHB (140°C., vacuum, 24 h) were charged into a 10 L Drais reactor and stirred for30 minutes to let the DHB dissolve. After cooling to 5° C., TDC wasadded while continuously stirring (250 rpm). After 50 minutes a samplewas taken, 1.8 L of NMP were added. The mixture was stirred for 30 min,another sample was taken and again 1.8 L of NMP were added. The mixturewas stirred for 30 min and the reactor was emptied through a bottomvalve. By applying this procedure, the first sample had a polymerconcentration of 7.4%, the second sample (after dilution with NMP) had aconcentration of 5% and the final product had a polymer concentration of4%. The relative viscosity of the reaction product was 3.43.

The polymerization procedure for the second batch was similar, exceptthat after 60 minutes a sample was taken and 4.0 L of NMP were added.The mixture was stirred for 30 min and then emptied. By applying thisprocedure, the first sample had a polymer concentration of 7.4% and thefinal product had a polymer concentration of 4%. The relative viscosityof the reaction product was 3.06. The polymerization batches were mixedprior to spinning.

COMPARATIVE EXAMPLE 1

Polymerization of PPTA para-phenyleneterephthalamide was carried outusing a 160 L Drais reactor. After sufficiently drying the reactor, 64 Lof NMP/CaCl₂ with a CaCl₂ concentration of 2.5 wt % were added to thereactor. Subsequently, 1522 g of PPD were added and dissolved at roomtemperature. Thereafter the PPD solution was cooled to 5° C. and 2824 gof TDC were added. After addition of the TDC the polymerization reactionwas continued for 45 min. Then the polymer solution was neutralized witha calcium oxide/NMP-slurry (780 g of CaO in NMP). After addition of theCaO-slurry the polymer solution was stirred for another 30 min. Thisneutralization was carried out to remove the hydrochloric acid (HCl),which is formed during polymerization. A gel-like polymer solution wasobtained with a PPTA content of 4.5 wt % and having a relative viscosityof 3.0 (in 0.25% H₂SO₄). This product has an etarel (η_(rel)) of 2.4 anda polymer concentration of 3.6% and was used to spin fibrids as well aspulp. Water was used as coagulant.

EXAMPLE 2 Fibrid and Pulp Making

The solutions of Example 1 and Comparative Example 1 were spun through ajet spinning nozzle (spinning hole 500 μm) at 20 L/h. Water was addedthrough a ring-shaped channel flowing perpendicular to the polymer flow.During spinning the polymer flow was kept constant while the coagulantpressure was changed for the different samples in order to vary the SR(°SR) of the product.

Pulp Spinning

The solutions of Example 1 and Comparative Example 1 were spun into pulpthrough a 1 hole jet spinning nozzle (spinning hole 350 μm). Thesolution was spun into a zone of lower pressure. An air jet wasseparately applied perpendicularly to the polymer stream throughring-shaped channels to the same zone were expansion of air occurred.Thereafter, the pulp was coagulated with water in the same zone by meansof applying a coagulant jet through ring-shaped channels under an anglein the direction of the polymer stream.

To spin the pulp with different SR values (°SR) the air pressure waskept constant while the polymer flow was varied. After spinning allsamples were washed with water.

The process and property data of fibrids and pulp obtained in Example 2are given in Table 1:

TABLE 1 Process parameters Properties Polymer Coagulant Coagulant SR DryPolymer Product solution pressure flow Airflow Fines Value SSA SolidsSample solution type Flow (L/h) (bar) (L/h) (Nm³/h) LL_(0.25) (%) (° SR)(m²/g) (%) A Example1 pulp 6 50 12 0.58 43.3 63 0.6 5.3 B Example1fibrid 20 50 0.72 25 67 0.5 7.3 C CompEx1 fibrid 20 30 0.84 25 42 2.25.6 D CompEx1 fibrid 20 50 0.74 26.3 65 2.6 4.8 E CompEx1 pulp 6 50 120.55 49.9 68 5.6 7.5 F CompEx1 pulp 18 50 12 0.62 42.7 46 3.9 6.9

EXAMPLE 3 Paper Making from Fibrids

Handsheets from 100% fibrids of samples A1 and B1-B4 and comparativeexamples D1-D4 and E1 with different grammage were made on a RapidKothen machine. The dewatered sheets were dried between two blottingpapers under vacuum (95° C., 1000 mbar, 20 min). Paper data are given inTable 2. Notice the lower calliper (paper thickness) and higherdensities for the papers of the invention in comparison to the referencepapers. TI (Tensile Index) is 3-5 times as high for the papers of theinvention as for the pulp-based reference papers when compared at thesame grammage. EAB is also higher for the papers of the invention.

TABLE 2 Properties of paper samples from fibrid Paper Grammage CalliperDensity EAB TI Sample (g/m²) (mm) (g/cm³) (%) (Nm/g) B1 99 0.168 0.594.3 85.2 B2 50 0.115 0.44 3.6 75.3 B3 29 0.073 0.39 3.7 72 B4 16 0.0580.28 2.5 41.6 D1 110 0.284 0.39 1.7 28.3 D2 52 0.193 0.27 1.7 19 D3 310.131 0.23 1.1 14.1 D4 16 0.092 0.17 1.6 8.1

EXAMPLE 4 Paper Making from Pulp

Handsheets from 100% pulp of samples A and E with a grammage of around100 g/m² were made on a Rapid Kothen machine using the same procedure asExample 3. Paper data are given in Table 3.

TABLE 3 Properties of paper samples from pulp Paper Grammage CalliperDensity EAB TI Sample (g/m²) (mm) (g/cm³) (%) (Nm/g) A1 110 0.265 0.4151.5 18.8 E1 117 0.296 0.395 1.05 9.5

EXAMPLE 5 Heat Treatment of Papers

To convert the above polybenzazole precursor paper to the polybenzazolepaper a heat treatment was performed under an inert atmosphere. Theprocedure was as follows: The samples were enclosed in an oven under anitrogen flow and heated with a heating rate of 5° C./min. When thetemperature of 440° C. was reached the samples were immediately takenout of the oven. Property data of the samples before and after heattreatment are given in Table 4. IR spectra of the samples were recordedon the Varian FTS-575c Infrared spectrometer equipped with theThunderdome ATR accessory. The spectra confirmed conversion to apolybenzoxazole paper with a conversion factor higher than 95%.

TGA experiments were carried out by means of a Setaram TGA/DSC 111,under nitrogen gas. The paper samples were first cut into pieces andthen put in Platinum (open) cells. The sample weight that was used wasbetween 10 and 20 mg. The samples were heated from 20° C. to 700° C. ata heating rate of 10° C./min. The onset of degradation Td was determinedby the temperature at which 1 weight percent weight loss is found. Incase of samples B5 and A1, Td was determined after complete conversionwhich occurred between 250 and 400° C. The results are denoted in Table4.

TABLE 4 Heat Grammage Calliper Density Td Paper Sample Type Treated(g/m²) (mm) (g/cm³) (° C.) EAB (%) TI (Nm/g) B5 fibrid No 102.2 0.1810.56 631 3 70 paper B6 fibrid Yes 99.8 0.151 0.66 626 3.6 80 paper A1pulp No 110 0.265 0.42 624 1.5 18.8 paper A2 pulp Yes 118 0.207 0.57 6181.8 16.8 paper D2 fibrid No 52 0.193 0.27 540 1.7 19 paper D3 fibrid Yes48 0.195 0.25 542 1.1 9 paper

EXAMPLE 6 Making Functionalized Paper

A precursor paper (paper sample B5), a polybenzoxazole paper obtained byheat treatment of the precursor paper (paper sample B6) and a PPTA paper(paper sample D1) were dyed with a reactive coloring agent (CibacronDark Blue S-GL; ex Ciba, Switzerland) according to the followingprocedure:

A solution of 6 grams of NaCl in 200 mL of demineralized water wasprepared at 80° C. After adding 0.4 g of Cibacron Dark Blue S-GL thesolution was stirred for 20 minutes and cooled down to 60° C. 4.3 gramsof Na₂CO₃.10H₂O were added and the solution was stirred for 30 minutesat 60° C. to obtain the dyeing fluid.

Samples B5, B6, and D1, each of 5 cm length and 1 cm width, weresubmerged into the dyeing fluid for 45 minutes at 60° C. andsubsequently rinsed in running water of about 50° C. for 10 minutes. Thesamples were neutralized in a 1% acetic acid bath and washed withrunning cold water for 15 minutes.

TABLE 5 Original color and color after dyeing of the paper samples PaperSample Original Color Color after dyeing B5 light green dark blue B6light brown light brown D1 light yellow light yellow with light bluestains

The present invention provides aromatic polyamides that arefunctionalized with a reactive functional group that can be used tofacilitate the conjugation of the aramids to a conjugation partner. Asshown in Table 5, a functionalized aramid (sample B5) shows excellentdyeability compared to the non-functionalized aramid D1 and to sampleB6, which does not contain reactive functional groups due to thecomplete conversion by heat treatment of B5 to B6, which has a fullyring closed polybenzoxazole structure.

1. A paper comprising at least one of a fiber, pulp, fibril, floc, andfibrid having a polybenzazole structure with a repeating unit of formula(I) and/or (II)

or its precursor structure with a repeating unit of formula (III):

wherein Ar¹ and Ar² are independently a para or meta aromatic grouphaving 4 to 12 carbon atoms, X and Y are the same or different andselected from O, S, and NH; and n is 0 or 1, and wherein the paper isfree or essentially free of non-extractable phosphorus compound.
 2. Thepaper of claim 1 obtainable by polymerizing about equimolar amounts ofmonomers having the formula ClOOC—Ar¹—COOCl and

wherein Ar¹ and Ar² are independently a para or meta aromatic grouphaving 4 to 12 carbon atoms, X and Y are the same or different andselected from O, S, and NH; and n is 0 or 1, to the precursor structurecontaining the repeating unit of formula (III),

wherein Ar¹, Ar², X, Y and n have the previously given meanings,optionally followed by cyclising the precursor structure to thepolybenzazole structure with the repeating unit of formula (I) and/or(II)

wherein Ar¹, Ar², X, and Y have the previously given meanings, in afluid which is free or essentially free from phosphoric acid.
 3. Thepaper of claim 1 wherein at least part of XH and/or YH isfunctionalized.
 4. The paper of claim 1 having a grammage from 1 to 16g/m².
 5. The paper of claim 1 comprising a mixture of at least one offiber, pulp, fibril, floe, and fibrid having the polybenzazole structureof formula (I) and/or (II), or the polybenzazole precursor structure offormula (III), and PPTA fibrid.
 6. A method for making the paper ofclaim 1 comprising polymerizing about equimolar amounts of monomershaving the formula ClOOC—Ar¹—COOCl and

wherein Ar¹ and Ar² are independently a para or meta aromatic grouphaving 4 to 12 carbon atoms, X and Y are the same or different andselected from O, S, and NH; and n is 0 or 1, to a precursor structurecontaining the repeating unit expressed by formula (III)

wherein Ar¹, Ar², X, Y and n have the previously given meanings,optionally followed by cyclising under heating in an inert atmospherethe precursor structure to a polybenzazole structure having a repeatingunit of formula (I) and/or (II)

wherein Ar¹, Ar², X, and Y have the previously given meanings, in afluid which is free or essentially free from phosphoric acid, followedby making a fiber, pulp, fibril, floc, or fibrid from the precursor orfrom the polybenzazole structure, followed by making the paper thereofby a conventional papermaking process optionally followed by one or moreof a calendering step, heating step, drying step, and functionalizationstep.
 7. The method according to claim 6 wherein at least part of the XHand/or YH groups of Ar² are functionalized by treating the monomerand/or the precursor structure and/or the polybenzazoe structure with afunctionalizing agent.
 8. A method for making the paper of claim 1comprising polymerizing about equimolar amounts of monomers having theformula ClOOC—Ar¹—COOCl and

wherein Ar¹ and Ar² are independently a para or meta aromatic grouphaving 4 to 12 carbon atoms, X and Y are the same or different andselected from O, S, and NH; and n is 0 or 1, to a precursor having therepeating unit of formula (III)

wherein Ar¹, Ar², X, Y and n have the previously given meanings, in afluid which is free or essentially free from phosphoric acid, followedby making a fiber, pulp, fibril, floe, or fibril from the precursor,followed by making the paper thereof in a conventional papermakingprocess, optionally followed by one or more of a calendering step,heating step, drying step, and functionalization step.
 9. The methodaccording to claim 7 wherein the paper obtained is heated under an inertatmosphere at a temperature allowing cyclization of the polybenzazoleprecursor having formula (III) to the polybenzazole comprising thestructure of formula (I) and/or (II).
 10. The method according to claim8 wherein at least part of the XH and/or YH groups of Ar² arefunctionalized by treating the monomer and/or the precursor structureand/or the polybenzazoe structure with a functionalizing agent.
 11. Amethod for making the paper of claim 6 comprising applying aconventional papermaking process further using at least one of fiber,pulp, fibril, floc, and fibrid having the structure IV

wherein the Ar¹ moieties have independently the previously givenmeaning.
 12. An electrical insulation material comprising the paper ofclaim
 1. 13. An electrical insulation material, a honeycomb structure,or a constructive material comprising as a component the paper of claim1.